The present invention relates generally to optical devices based on liquid crystals and more specifically to an optical device which is based on ferroelectric liquid crystal (FLC) materials and is generally free of chevron structures.
Optical devices based on surface-stabilized ferroelectric liquid crystal (SSFLC) structures have gathered considerable scientific and commercial interest in recent years. As is well known in the art, when confined between a pair of substrates, FLC molecules tend to become oriented in layers, which are called smectic layers. In the Smectic C phase, the long axis, or director, of each FLC molecule is generally tilted at an angle with respect to the smectic layers.
Turning now to the drawings, wherein like components are indicated by like reference numbers throughout the various figures, attention is immediately directed to FIG. 1A, which illustrates the smectic layer structure of an FLC material, generally indicated by reference number 10. As can be seen in FIG. 1A, FLC molecules 20 of FLC material 10 form smectic layers 22, with the director of each FLC molecule being tilted at an angle θ0 with respect to smectic layers 22. Although this angle θ0 with respect to the smectic layers remains constant among the FLC molecules, the director orientation of the FLC molecules rotates on a conical surface 24 with cone angle θ0 through the successive layers. In other words, FLC molecules 20 have a natural tendency to form a helical structure along an axis 28 with each FLC molecule lying in a slightly displaced position on the conical surface 24 relative to FLC molecules in adjacent layers. It should be noted that each FLC molecule possesses a polarization 26 which is perpendicular to the director of the molecule. The presence of polarization 26 becomes important when an electric field is applied across the FLC material, as will be described hereinafter at an appropriate point in the discussion.
Turning now to FIG. 1B, the suppression of the formation of helical structures in an SSFLC device is illustrated. FLC material 10 is confined between parallel substrates 32. When the spacing between the substrates is reduced to a few microns, the helical rotation of the director through the layers is prevented by the interaction of the molecules with the inner surfaces of the confining substrates. As shown in FIG. 1B, FLC molecules 20 in an SSFLC configuration become confined to one of two positions, A and B, which are parallel to the substrates on a cross section parallel to the substrate surfaces of a conical surface 24′ with cone angle θ. The cone angle θ is generally smaller than θ0 due to the interaction of the FLC molecules with the substrates. Smectic layers 22′ are also of a slightly different width in comparison to smectic layers 22 of FIG. 1A due to the difference in cone angles. The projection of the director of each FLC molecule 20 onto the plane of substrate 32 is shown as line 20′. It should be noted that each projection, represented by line 20′, makes an angle θ with respect to a line 28′, which is a projection of axis 28 onto the plane of substrate 32. As shown in FIG. 1B, the projection of the director of an FLC molecule in position B makes an angle 2θ with respect to the projection of the director of an FLC molecule in position A.
The two molecular positions A and B give rise to two possible optical states in the SSFLC device. Due to the symmetry of the system, the FLC molecules are generally stable in either of these optical states, and, as a result, the SSFLC device produces bistable optical states. The molecular positions A and B are respectively associated with the DOWN orientation of polarization 26 and the UP orientation of polarization 26 indicated by arrows 34 and 36, respectively. Since polarization 26 of each FLC molecule 20 tends to align along the direction of an electric field applied across the FLC material, the molecular position of the FLC molecule can be switched between positions A and B on conical surface 24′ by applying an electric field in the UP direction or in the DOWN direction. In this way, the SSFLC device can be switched between the two optical states by applying an electric field across the FLC molecules between the substrates in the UP and DOWN directions.
Conventional SSFLC structures have certain inherent drawbacks. One such drawback is the fact that the device can only be in one of two optical states. Unlike nematic liquid crystal (NLC) devices, which is capable of analog operation by generating a continuum of optical states between a minimum and a maximum state according to the magnitude of the electric field applied across the NLC material, SSFLC devices are limited to binary operation in which one of two optical states is generated according to whether a positive or negative magnitude electric field is applied across the FLC material. Since molecular positions between positions A and B are not stable, optical states between the two stable states are not controllably accessible in a conventional SSFLC device. This inflexibility can be problematic especially in applications in which analog operation is desired.
Still another problem seen in the conventional SSFLC device is the presence of chevron structures. As commonly known in the art, chevron structures form in SSFLC devices due to the shrinkage of the smectic layers during the transition from a Smectic A (Sm A) phase to a Smectic C (Sm C) phase. Generally, in the assembly of a new SSFLC device, an FLC material is injected into the space between confining, parallel substrates at an elevated temperature such that the FLC material is in an isotropic phase. The SSFLC device is then gradually cooled such that the FLC material transitions from the isotropic phase through nematic and smectic phases. The smectic layers normally form when the FLC material is in the Sm A phase while the device temperature is still higher than room temperature. In the Sm A phase, the smectic layers are formed in the FLC material and the directors of the FLC molecules align perpendicularly to the smectic layers. As the SSFLC device is further cooled to room temperature the FLC material transitions to a chiral Smectic C (Sm C*) phase, in which the directors of the FLC molecules become tilted with respect to the smectic layers and are aligned on the conical surfaces. The smectic layers shrink slightly during the transition from the Sm A phase to the Sm C* phase due to the tilting of the FLC molecules with respect to the smectic layers. Due to conservation of mass and the structural boundary conditions, the shrinkage of the smectic layers results in the formation of chevron structures. The discontinuities between distinct domains of chevron structures pointing in opposing directions are optically visible as a zigzag pattern in the SSFLC device. Consequently, the presence of chevron structures gives rise to nonuniformity in the optical state produced by the device as a whole.
Prior efforts to eliminate the occurrence of chevron structures have included the application of an AC voltage pretreatment to the SSFLC device. The application of a predetermined AC voltage to the SSFLC device after the FLC material has been injected between the substrates has the effect of “kicking” the FLC molecules into uniform alignment, thus straightening the smectic layers and eliminating the chevron structures. The use of the AC voltage pretreatment adds an extra step to the manufacturing process of the SSFLC device, thus lead to additional costs associated with the device assembly. Furthermore, the use of the AC voltage pretreatment often results in smectic layer undulation in the plane of the substrate, thus yielding an SSFLC device with lower contrast and generally poor performance.
Another approach for eliminating chevron structures is the use of special FLC materials that do not have a Sm A phase. These special materials have the property that no shrinking of the smectic layers takes place during the phase transition into the Sm C* phase, hence the occurrence of chevron structures is prevented. Unfortunately, such FLC materials are not commonly used in commercial applications because it is generally more difficult to achieve uniform alignment of these special FLC materials in comparison to conventional FLC materials. Also, there are fewer examples of these materials available commercially.
Another problem with the conventional SSFLC device is the instability of the two optical states associated with molecular positions A and B due to the fact that the FLC molecules adjacent to the inner surface of the substrates do not switch and also due to the presence of chevron structures.
Yet another drawback of the conventional SSFLC device is variation in the optical retardance of the device during switching between the two optical states. Referring again to FIG. 1B, although the helical structures are suppressed, FLC molecules 20 still tend to rotate about conical surface 24′ in switching from position A to position B and vice versa in the direction of residual twist. This characteristic is commonly referred to as cone switching of FLC molecules. Since optical retardance of the SSFLC device is generally proportional to the length of the projection of the FLC molecule director onto the substrate plane, the symmetry of the system provides that the optical retardance of the SSFLC device when the FLC molecules are in position A is the same as that when the FLC molecules are in position B. However, the optical retardance changes during cone switching since the length of the projection of the FLC molecule director varies as the FLC molecule rotates around the cone. Although the switching between positions A and B normally takes 100 μs or less, this effect of optical retardance variation can be problematic in certain applications requiring constant optical retardance.
In an effort to counter some of the aforedescribed problems of the conventional SSFLC device, Kitayama discloses in U.S. Pat. No. 4,778,259 a method for stabilizing the optical states of an SSFLC structure. By offsetting the alignment axes of the top and bottom substrates by a small, non-zero angle, an additional molecular twist is introduced in the FLC material to counter the aforementioned, inherent twist of the FLC, thus further stabilizing the optical states of the resulting SSFLC device. Kitayama also uses an AC voltage treatment to achieve full cone switching (i.e., use the full cone angle of θ0 rather than θ<θ0). In U.S. Pat. No. 5,172,257, Patel takes the idea of the offset twist further by orienting the alignment axes of the alignment layers formed on the top and bottom substrates perpendicularly to one another. The alignment layers provide “strong anchoring” of the FLC molecules adjacent to the alignment layers such that the FLC molecules align in one of the two stable positions in parallel to the respective alignment axes with a 90° angle between the molecules near the top surface and those near the bottom surface. Thus, the natural twist of the FLC molecules is enhanced and an FLC device analogous to a twisted nematic device is achieved. The device disclosed in Patel is capable of exhibiting grayscale by continuously varying the applied electric field, thus changing the twist of the FLC molecule between the top and bottom substrates. In other words, the FLC device according to Patel is an analog device, which is capable of exhibiting continuously variable, not bistable, optical states. However, it is submitted that the device disclosed in Patel has a number of disadvantages. A special FLC material with θ0=45° is required and the overall device tends to exhibit multiple domains that must be made uniform by use of an AC voltage pre-treatment. In addition, neither Kitayama nor Patel specifically addresses the problems of chevron structures and optical retardance variation in SSFLC devices.
As will be seen hereinafter, the present invention provides a heretofore unseen and highly advantageous approach with regard to achieving an FLC-based optical device while eliminating the problems present in prior art display systems.